For many years, the binders generally used to manufacture engineered wood composite products such as plywood, oriented strand board (OSB), particleboard, or medium density fiberboard have been resins based on urea-formaldehyde, phenol-formaldehyde, or melamine-formaldehyde. These formaldehyde-based resins are relatively low in cost and provide broad processing latitude in mills with different types of production line configurations. In many of the known processes for the production of wood composites, wood pieces, chips, particles, or fibers are treated with binder and then “cold-pressed” into a mat or cake that moves along a conveyor to a high temperature pressing assembly where the loose mat is further compressed into a thin panel and the adhesive properties of the binder are activated.
One of the disadvantages of formaldehyde-based resins is that over time these resins emit formaldehyde into the atmosphere at levels that may have deleterious health consequences when used in confined areas with limited ventilation.
In 2007, the California Air Resources Board (CARB) approved the Airborne Toxic Control Measure (ACTM) requirements for fabricators of composite wood products that are bonded with a resin to produce wood-based panels for interior use. ACTM targets hardwood/plywood composites, particleboard, and medium density fiberboard in which the predominant binder is a formaldehyde-containing resin. Starting with Phase 1 in January of 2009, ACTM limits the level of formaldehyde released from finished goods such as furniture, cabinets, doors, or other items used in the construction of new homes. Full implementation of ACTM takes place in January 2011 for particleboard and medium density fiberboard.
Depending upon the type of mill operation, e.g., whether a continuous or discontinuous process is employed in the production of particleboard, the degree of cohesiveness of the mat formed from the wood particles or “furnish” needed to maintain its shape and integrity as it moves along the conveyor prior to the compression step may vary. Traditional urea-formaldehyde binders are known to impart sufficient cold tack or wood particle cohesion to the mat that relatively high production rates can be achieved. However, isocyanate binders do not exhibit tack comparable to existing commercial systems. Therefore mat integrity is not maintained under some of the more severe continuous mill processing conditions. In such situations, there may be gaps along the conveyor as the mat is lowered from one level to another on its way to the final press. The mat must be sufficiently strong that it will not collapse under its own weight as it extends over the gap before coming in contact with the next belt.
There have been a variety of attempts to address the tack deficiency associated with replacement of urea-formaldehyde binders in wood composites with isocyanate adhesives.
U.S. Pat. No. 3,931,088 teaches that a composition composed of an aqueous solution of polyvinyl alcohol (PVA) and a hydrophobic solution of an isocyanate compound is useful as an adhesive for the production of plywood or particleboard. The disclosed adhesive mixture in this patent is said to provide “an excellent initial adhesive strength”. A solvent is used to dissolve the isocyanate compound employed in U.S. Pat. No. 3,931,088, to uniformly disperse the isocyanate compound in the PVA emulsion, and to protect the isocyanate groups from direct contact with water prior to the heat-pressing step. No mention of “tack” is made in this patent and no data comparing the “initial adhesive strength” of the disclosed isocyanate-containing adhesive with any formaldehyde resin system is presented.
Another approach to using isocyanate binders as alternatives to urea-formaldehyde resins is disclosed in U.S. Pat. No. 4,279,788 which teaches that aqueous polyisocyanate-lignin adhesives are useful in the manufacture of wood composites. According to the teachings of U.S. Pat. No. 4,279,788, water in the adhesive composition serves an important plasticizing function, facilitates intimate contact between chip surfaces and provides the “pre-pressing tack needed for cold press mat consolidation”. However, no examples or data are presented to demonstrate these alleged advantages. Further, it is not clear to what extent, if any, the cold tack of the disclosed polyisocyanate-lignin adhesive is comparable to or exceeds that of conventional urea-formaldehyde resins.
U.S. Pat. Nos. 5,180,770 and 5,214,081 each discuss the lack of sufficient “green tack” of polyisocyanate binders used in wood composite production. Each of these patents discloses a binder composition composed of an emulsion containing at least one polyisocyanate and at least one water-dispersed acetovinylic resin having a glass transition temperature of less than 5° C. In one embodiment of the process disclosed in this patent, the polyisocyanate may be added to an aqueous dispersion of an acetovinylic polymer to produce an emulsion that is applied to the wood particles. In an alternative method, a polyisocyanate emulsion and an aqueous acetovinylic polymer dispersion are added separately to the wood particles. In either method, the amounts of polyisocyanate emulsion and aqueous acetovinylic polymer dispersion and the processing conditions are such that a mat is produced with green tack cohesion values greater than 1 kg. The green tack is measured by forming 50 mm diameter pellets using 30 grams of the wood pieces or particles treated with the emulsion and applying a load of 500 kg to the treated pellets for 30 seconds. The pellets are tested in a dynamometer and the load at break is observed. No examples comparing the disclosed polyisocyanate/acetovinylic binders with the known urea-formaldehyde resins being replaced are given.
The prior art has not therefore disclosed a polyisocyanate binder composition that can be used to replace resins containing formaldehyde in the production of wood composite products which matches or exceeds the cold tack performance of the known formaldehyde-containing resins.